Method for determining optimal location and value of dispersion compensation modules in an optical network

ABSTRACT

A method for determining optimal locations and values of dispersion compensating modules (DCMs) in an optical network is provided. The method comprises repeatedly evaluating possible combinations of DCM values and locations and adding to the combination having the lowest score until a solution of DCMs is formed that satisfies the dispersion limits of the network. This method provides an optimal solution with, for example, the lowest value of DCMs necessary to meet dispersion specifications. In one embodiment, the method for determining the optimal location and value of the DCMs uses a priority queue to store the different combinations of DCM values and locations. Modifications to the method include variations in the score function, for example to minimize the total cost of the DCMs. In another embodiment, a series of priority queues are used to improve the efficiency of the method by reducing the amount of processing required to sort the priority queues.

RELATED APPLICATIONS

[0001] This is a CONTINUATION APPLICATION of, and claims benefit of,U.S. patent application Ser. No. 10/443,965 to Ng, E.K.H., filed on 23May 2003, and entitled “Method for Determining Optimal Location andValue of Dispersion Compensation Modules in an Optical Network”.

[0002] This application also claims priority from U.S. ProvisionalPatent Application Ser. No. 60/402,563 to Ng, et al, filed on 12 Aug.2002, entitled “Method for Determining an Optimal Location and Value ofDispersion Compensation Module (DCM) in an Optical Network”; and isrelated to U.S. patent application Ser. No. 10/273,858 to Ng, et al,filed on 21 Oct. 2002, entitled “Method and System for DeterminingLocation and Value of Dispersion Compensating Modules in an OpticalNetwork”.

FIELD OF THE INVENTION

[0003] The present invention relates generally to optical networks, andin particular to a method for determining optimal location and value ofdispersion compensating modules (DCMs) in optical networks.

BACKGROUND OF THE INVENTION

[0004] Dynamic networks have grown in size over the past decades fromlocal area networks (LANs) to metropolitan area networks (MANs) to widearea networks (WANs), as user demand for connectivity has increased. Newdesign issues have arisen and continue to arise as these networks becomelarger and more complex, necessitating the use of components such asdispersion compensating modules (DCMs) and optical amplifiers.

[0005] Determining the location and value of DCMs in MANs is a designissue that has arisen since the growth in the size of MANs has reachedthe degree that dispersion compensation has become necessary in MANs.Minimizing the total number and values of DCMs in the network is themotivating factor in DCM placement methods for MANs because smaller,dynamic networks, such as MANs, are cost-sensitive.

[0006] Currently, there are existing methods for determining thelocation and value of DCMs in a MAN whereby DCMs are placed on aselected number of fiber spans in the network. These methods areperformed manually using the intuition and experience of a designer toselect the locations and values of DCMs in the networks, as isillustrated by the following document.

[0007] An optical university project by L. Chrotowski, C. Mateus, F. Mo,and L. Zhou at the University of California, Berkeley dated Dec. 17,2001 and entitled “Optical Network Design of a Metro Ring” discloses amethod for DCM design in a metro ring network involving quantifying thedegree of eye closure on a signal, which is used as the factor uponwhich the DCM placement is dependent. The placement itself however isdetermined heuristically by the designers, who attempt to minimize thetotal number of DCMs in the network by determining placement of justenough DCMs so that the network is operating within desired conditions(in this case to a maximum value of eye closure).

[0008] However, real-world MANs are topologically complex and often takethe form of rings or meshes that may include coupled lightpaths. Withincreasing size and complexity of MANs, the manual, heuristic methods ofDCM placement become impractical and inefficient.

[0009] A systematic method for determining location and value of DCMsallows efficient placement of DCMs in a variety of network topologies,as illustrated in the following patent application. U.S. Pat. No.5,559,920 to Ng et al. filed Sep. 24, 1996 and entitled “Method andsystem for determining location and value of dispersion compensatingmodules in an optical network” discloses a DCM placement procedure thatcomprises evaluating possible DCM values and locations and successivelyadding selected combinations to the network until the dispersion limitsof the network are met. The method, however, does not guaranteeoptimality, optimality being the ability to maximize or minimize a givenvariable such as total dispersion, number, or cost of DCMs in thenetwork.

[0010] Therefore, there is a need in the industry for the development ofa systematic method for determining the location and value of DCMs in anoptical network that would provide an optimal solution.

SUMMARY OF THE INVENTION

[0011] Therefore there is an object of the invention to provide a methodfor determining the optimal location and value of DCMs in an opticalnetwork that would avoid or minimize the above-mentioned drawbacks.

[0012] According to one aspect of the invention, there is provided amethod for determining the optimal location and value of one or moreDCMs in an optical network, comprising the steps of:

[0013] (a) determining a lightpath topology in the network;

[0014] (b) introducing and initializing a data structure having multipleentries, each entry in the data structure being used for storing DCMlocations and values in the network and a score measuring theeffectiveness of dispersion compensation in the network by the storedDCMs;

[0015] (c) extracting the entry from the data structure, which has thelowest score and determining if the effective dispersion on thelightpaths in the network having the stored DCMs from the extractedentries are substantially zero, the effective dispersion being an amountof dispersion accumulated along a lightpath that exceeds the maximumpositive dispersion value Pos_Disp_Limit specified for the network;

[0016] (d) if the effective dispersions on a lightpath is notsubstantially zero, expanding the extracted entry into multiple entriesby adding available combinations of DCM location and value to theextracted entry;

[0017] (e) calculating a score for each expanded entry and discardingthose entries that cause the accumulated dispersion on any lightpath tobe less than the maximum negative dispersion limit Neg_Disp_Limit of thenetwork;

[0018] (f) inserting the expanded entries into said data structure; and

[0019] (g) repeating the steps (c) to (f) until the effectivedispersions are substantially zero for the extracted entry in the step(c).

[0020] Advantageously, the step (d) of expanding comprises expanding theextracting entry into multiple entries by adding every availablecombination of DCM location and value to the extracted entry.

[0021] The step of introducing and initializing the data structurecomprises introducing and initializing the data structure which is apriority queue, including maintaining the entries in an ascending orderaccording to the score; and the step (c) of extracting the entry withthe lowest score comprises extracting the first entry from the priorityqueue.

[0022] A method may further comprise the step of maintaining thepriority queue in an ascending order according to the score, comprisingsorting the priority queue, the step being performed after the step (f).

[0023] Conveniently, the step (f) may comprise inserting the expandedentries into the priority queue so that the priority queue maintains theascending order according to the score.

[0024] Beneficially, the step of determining the lightpath topologycomprises:

[0025] identifying lightpaths in the network;

[0026] assigning lightpath identification numbers to the lightpaths; and

[0027] identifying fiber spans over which the lightpaths are laid.

[0028] The step of identifying lightpaths in the network may compriseidentifying all lightpaths in the network including protectionlightpaths and reconfigurable lightpaths.

[0029] The step (e) of calculating the score conveniently comprisescalculating the score to be equal to the sum of:

[0030] the total value of the DCMs stored in the entry; and

[0031] the remaining effective dispersion in the network divided by thenumber of lightpaths having remaining effective dispersion.

[0032] For example, the step (e) of calculating the score may comprisecalculating the score to be equal to the sum of:

[0033] Σw_(i)·DCM_(min), wherein DCM_(min) is the smallest DCM value tobe used in the network, and w_(i) is the weight factor for thecorresponding DCM_(i) stored in each expanded entry; and${{g(x)} \cdot {\min \left( \frac{w_{i} \cdot {DCM}_{\min}}{{DCM}_{i}} \right)}},$

[0034] wherein g(x) is the remaining effective dispersion in the networkdivided by the number of lightpaths having remaining effectivedispersion, and$\min \left( \frac{w_{i} \cdot {DCM}_{\min}}{{DCM}_{i}} \right)$

[0035] is the smallest value of$\left( \frac{w_{i} \cdot {DCM}_{\min}}{{DCM}_{i}} \right).$

[0036] Advantageously, the step (c) of determining if the effectivedispersions are substantially zero comprises measuring the effectivedispersions in units of distance.

[0037] If required, a method may further comprise the step ofdetermining alternative DCM locations such that the transfer of the DCMto the alternative location does not change the accumulated dispersionalong any lightpath in the network.

[0038] In a method described above, the step of introducing andinitializing the data structure may comprise introducing andinitializing the data structure, which is a series of priority queues,the series being maintained in an ascending order according to the scoreof the first entry of the priority queues, and the step (c) ofextracting the entry with the lowest score may comprise extracting thefirst entry from the first priority queue in the series of priorityqueues.

[0039] Conveniently, the method provides the optimal location and valueof one or more dispersion compensating modules (DCMs) in an opticalnetwork, wherein the optimal location is being defined as providing oneor more of the following:

[0040] an optimal total dispersion in the network;

[0041] a minimal number of DCMs in the network; and

[0042] a minimal cost of DCMs in the network.

[0043] The methods for determining the optimal location and value ofDCMs in an optical network of the embodiments of the invention provide asystematic procedure that is efficient and applicable to a variety ofnetwork topologies.

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Embodiments of the invention will now be described, by way ofexample, with reference to the accompanying drawings in which:

[0045]FIG. 1 is an exemplary optical network used for illustratingmethods for determining the placement and value of DCMs according toembodiments of the invention;

[0046]FIG. 2 is a flowchart illustrating the steps of the method fordetermining the optimal placement and value of DCMs according to theembodiments of the invention;

[0047]FIG. 2A is a flowchart illustrating the step 202 of determiningthe lightpath topology of an optical network in the method of FIG. 2 inmore detail; and

[0048]FIG. 3 is a flowchart illustrating the steps of the method fordetermining the optimal placement and value of DCMs according to thefirst embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0049] An exemplary optical network 10 is illustrated in FIG. 1 ascomprising a plurality of, in this example seven, nodes 12 identifiedindividually as Nodes “A” through “G” (namely, nodes “A”, “B”, “C”, “D”,“E”, “F”, “G”), which are coupled together via fiber spans 14 identifiedindividually by the fiber identification numbers (IDs) “1” through “7”and represented by straight solid lines. Lightpaths 16 are representedby curved solid lines with arrows indicating the direction of a networksignal traveling from a source node to a destination node.

[0050] As is known in the art, such an optical network may include anarbitrary number of nodes 12 and lightpaths 16, and each of the fiberspans 14 may have different lengths and thus different dispersions.Also, such an optical network 10 may have other arrangements of nodes 12and other lightpaths 16 through the nodes 12, such as mesh or startopologies. Accordingly, FIG. 1 serves merely to illustrate one form ofoptical network for the purpose of describing embodiments of theinvention.

[0051]FIG. 2 is a flowchart 200 illustrating the steps of the method fordetermining the optimal location and value of dispersion compensatingmodules (DCMs) in the optical network 10 according to embodiments of theinvention.

[0052] Upon start 201, the procedure 200 determines the lightpathtopology of the network (step 202). Determining the lightpath topology(step 202) comprises the steps illustrated in FIG. 2A of identifyingeach lightpath 16 in the optical network 10 (step 202 a), assigning alightpath identification number (ID) to each lightpath 16 (step 202 b),and identifying the fiber spans and noting the fiber IDs of those fiberspans over which each lightpath 16 is laid (step 202 c).

[0053] After determining the lightpath topology, the flowchart 200proceeds with introducing a data structure (step 204), each entry of thedata structure storing different selections of DCM locations and valuesand a score. The stored DCM locations and values constitute a partialsolution of DCMs for the network and the score measures theeffectiveness of the stored DCMs in compensating for dispersion in thenetwork. The data structure is initialized to, for example, a singleentry with no DCMs, representing the initial network 10 without anyDCMs, and a score equal to the total effective dispersion of alllightpaths in the network.

[0054] The effective dispersion on a lightpath is defined as the amountof dispersion accumulated along the lightpath that exceeds the maximumpositive dispersion limit Pos_Disp_Limit of the network. The maximumpositive dispersion limit Pos_Disp_Limit is derived from the specifiedchromatic dispersion limit of the transceivers in the network 10, as isthe complementary maximum negative dispersion limit Neg_Disp_Limit.Dispersion parameters used in the method of FIG. 2, such as effectivedispersion and Pos_Disp_Limit, may be specified in units of distance, asopposed to units of dispersion, if the dispersion coefficient isconstant for all spans of fiber 14 in the network 10, as is the case inthe exemplary network 10 of FIG. 1.

[0055] After the step 204, the procedure 200 extracts that entry in thepriority queue having the lowest score (step 206) and determines if theeffective dispersion along any lightpath 16 in the network 10 issubstantially zero when the DCMs stored in the extracted entry arepresent on the network 10 (step 208).

[0056] If an effective dispersion is substantially zero with theextracted entry (exit “Yes” from the step 208), then the network 10meets dispersion specifications with the DCMs stored in that extractedentry and the optimal DCM placement procedure 200 is finished (step299). If the effective dispersion is positive with the extracted entry,i.e. not substantially zero (exit “No” from the step 208), then theflowchart 200 proceeds to the step 210.

[0057] In the step 210, the flowchart expands the entry extracted in thestep 206 into multiple entries by adding every available combination ofDCM location and value to that entry. The available DCM locations areeach of the fiber spans 14 in the network 10, namely fiber spans withIDs “1” through “7”. The available DCM values are those specified bydesign. For the exemplary network 10 of FIG. 1, the available DCM valuesare “40 km”, “60 km”, and “80 km”. Thus the initial entry of the datastructure with no DCMs is expanded into 21 entries, which are the 21different combinations of DCM location and value.

[0058] The flowchart 200 then calculates a score for each of theexpanded entries (step 212). In calculating the score, if the DCMsstored in the expanded entry are found to cause over-compensation ofdispersion, then that expanded entry is discarded. Over-compensation ofdispersion is the amount of accumulated dispersion less than the maximumnegative dispersion limit Neg_Disp_Limit that results from the placementof DCMs in the network.

[0059] After calculating a score for each expanded entry, the flowchart200 inserts the expanded entries back into the data structure (step 214)and then returns to the step 206 of extracting the entry with the lowestscore.

[0060] As mentioned above, optimality is defined as the ability tomaximize or minimize a given variable such as total dispersion, number,or cost of DCMs in the network. In the embodiments of this invention,the variable to be optimized is quantified by the score that is assignedto each entry in the data structure, each entry being a partial solutionof DCMs for the network. Thus, the score function is defined such thatthe optimal solution has the lowest score of all possible solutions.

[0061] The score may be defined as the sum of two terms, one term h(x)representing the present effectiveness of the DCMs in the entry and theother term g(x) estimating the remaining required effectiveness of theDCMS. For example, if the variable to be optimized is the total value ofDCMs in the solution, the score may be defined as:${f(x)} = {{{h(x)} + {g(x)}} = {{\sum{DCM}_{i}} + \frac{\sum{eff\_ disp}_{i}}{\# {\_ lightpaths}}}}$

[0062] where ΣDCM_(i) is the total value of DCMS stored in the entry,Σeff_disp_(i) is the total remaining effective dispersion in thenetwork, and #_lightpaths is the number of lightpaths having remainingeffective dispersion.

[0063] According to the flowchart 200 of FIG. 2, the optimal solution isthe first solution that is determined by the flowchart 200. It will beproven by way of contradiction that the first solution that isdetermined by the flowchart 200 is always the optimal solution.

[0064] Assume that a solution S is optimal, however, another solution S′is determined first. For the solution S′ to be determined first, it musthave a lower score than S or any partial solution p_S of S. If thesolution S′ has a lower score than S, then S cannot be the optimalsolution. Thus, for the solution S′ to be determined first:

f(S′)<f(p _(—) S)

[0065] In the definition of the score, the first term h(x) is the sum ofthe DCM values stored in the partial solution. This term increases invalue in successive expansions of the entry as DCMs are added to expandthe entry, thus h(S)≧h(p_S).

[0066] The second term g(x) is the average remaining effectivedispersion, which decreases with successive expansions of the entry asadded DCMs decrease the effective dispersion, thus g(S)≦g(p_S). Thissecond term is an exact or under-estimate of the actual remainingrequired DCM value, as will be shown below, and thus:

g(p _(—) S)−g(S)≦h(S)−h(p _(—) S)

[0067] which is equivalent to:

h(S)+g(S)>h(p _(—) S)+g(p _(—) S)

[0068] so that

f(S)≧f(p _(—) S)

[0069] In summary, it has been shown that for S′ to be determined as thefirst solution:

f(S′)<f(p _(—) S)≦f(S)

[0070] However, as mentioned above, a solution that is optimal has thelowest score of all possible solutions. Thus, if f(S′)≦f(S) then Scannot be the optimal solution, or conversely, the solution S′ cannot bedetermined before the optimal solution S.

[0071] It will now be shown that the second term${g(x)} = \frac{\sum{eff\_ disp}_{i}}{\# {\_ lightpaths}}$

[0072] is an exact or under-estimate of the actual remaining requiredDCM value. When lightpaths are not coupled, the required DCM value isthe total effective dispersion over all lightpaths, so the second termg(x) being an average of the effective dispersions is an under-estimate.When the lightpaths are coupled, the required DCM value is at least thelargest effective dispersion on one lightpath in order to eliminateeffective dispersion on that lightpath, and again the average is anunderestimate.

[0073] The only conditions that the second term must satisfy are that itbe an estimate of the remaining required DCM value and that it be anexact or under-estimate of the actual remaining required DCM value.Thus, the second term g(x) may also be, for example, the maximumeffective dispersion on any one lightpath 14 in the network 10.

[0074] A method of determining the optimal value and location of DCMs inan optical network according to a first embodiment of the invention isillustrated in FIG. 3. In the first embodiment, the data structure ofFIG. 2 is a priority queue that is maintained in ascending orderaccording to score, and the step 206 of extracting the entry with thelowest score comprises simply extracting the first entry in the priorityqueue (step 306). An additional step 316 is performed after the step 314and comprises sorting the priority queue in ascending order according toscore.

[0075] Thus, a method for determining the value and location of DCMs inan optical network is provided that is systematic, may be applicable toa variety of network topologies, and provides an optimal solution.

[0076] This method may be applied to protected networks havingadditional fiber spans forming protection lightpaths between nodes, andto reconfigurable networks having multiple reconfigurable lightpathsbetween nodes. In the step 202 a of identifying all lightpaths in thenetwork, the protection lightpaths and the reconfigurable lightpaths areidentified along with the working lightpaths. Thus, the method ensuresthat all lightpaths, including the protected lightpaths andreconfigurable lightpaths, meet dispersion specifications.

[0077] In a modification to the method of the first embodiment, the step316 of sorting the priority queue is absent/removed, and the step 314 ofinserting the expanded entries into the priority queue is modified tocomprise inserting the expanded entries so that the priority queuemaintains the ascending order according to score.

[0078] Thus, a method for determining the value and location of DCMs inan optical network is provided that does not require repeated sorting ofthe priority queue.

[0079] In another modification to the method of the first embodiment,the definition of the score includes weight factors applied to each DCMvalue to facilitate optimization of, for example, total monetary cost,insertion loss, or number of DCM cards. The weight factor is defined asthe ratio of that characteristic that is being optimized for each of theDCMs relative to the DCM with the smallest value available. For example,if the variable to be optimized is the total monetary cost of the DCMS,and DCMs with values “40 km”, “60 km”, and “80 km” have costs “$4000”,“$5600”, and “$7200”, then the ratios of the cost of the DCMs aredetermined to be “1.0”, “1.4”, and “1.8” respectively and are used asthe weight factors.

[0080] The definition of the score including the weight factors may bedefined as, for example:${f^{\prime}(x)} = {{{h^{\prime}(x)} + {g^{\prime}(x)}} = {{\sum{w_{i} \cdot {DCM}_{\min}}} + {{g(x)} \cdot {\min \left( \frac{w_{i} \cdot {DCM}_{\min}}{{DCM}_{i}} \right)}}}}$

[0081] where DCM_(min) is the smallest value of DCM available, and w_(i)is the weight factor of each DCM stored in the entry.

[0082] Thus, a method for determining the value and location of DCMs inan optical network is provided that optimizes variables such as totalcost, insertion loss, or number of DCMs cards in the network.

[0083] In yet another modification to the method of the fist embodiment,alternative fiber spans for the DCM location are determined such thatthe transfer of the DCM to the alternative fiber span does not changethe accumulated dispersion of any lightpath 16 in the network 10. Thisstep is performed upon exit “Yes” from the step 308 of determining ifthe effective dispersion is positive and is performed, e.g., accordingto the method detailed in U.S. patent application Ser. No. 10/273,858 toNg et al. filed Oct. 21, 2002 and entitled “Method and System forDetermining Location and Value of Dispersion Compensating Modules in anOptical Network”.

[0084] Thus, a method for determining the value and location of DCMs inan optical network is provided that is more flexible by determiningalternative locations of DCMs.

[0085] According to a second embodiment of the invention, the datastructure of FIG. 2 is a series of priority queues, the series beingmaintained in ascending order according to the score of the first entryof each priority queue. Each priority queue is formed in the step 214when the expanded entries are inserted as a new priority queue, which issorted in ascending order according to score, into the data structure.Thus, the step 206 of extracting the entry with the lowest scorecomprises extracting the first entry from the first priority queue ofthe series of priority queues.

[0086] Thus, an efficient method for determining the optimal value andlocation of DCMs in an optical network is provided.

[0087] It is apparent to those skilled in the art that there are manyvariations of the present invention that retain the spirit of theinvention. Thus it is intended that the present invention cover themodifications, variations, and adaptations of this invention providedthey fall within the scope of the following claims.

What is claimed is:
 1. A method for determining the optimal location andvalue of one or more dispersion compensating modules (DCMs) in anoptical network for compensating dispersion in the optical network,comprising the steps of: (a) determining a lightpath topology in thenetwork; (b) introducing and initializing a data structure havingmultiple entries, each entry in the data structure being used forstoring DCM locations and values in the network and a score measuringthe effectiveness of dispersion compensation in the network by thestored DCMs; (c) extracting the entry from the data structure, which hasthe lowest score and determining if the effective dispersion on thelightpaths in the network having the stored DCMs from the extractedentries are substantially zero, the effective dispersion being an amountof dispersion accumulated along a lightpath that exceeds the maximumpositive dispersion value Pos_Disp_Limit specified for the network; (d)if the effective dispersions on a lightpath is not substantially zero,expanding the extracted entry into multiple entries by adding availablecombinations of DCM location and value to the extracted entry; (e)calculating a score for each expanded entry and discarding those entriesthat cause the accumulated dispersion on any lightpath to be less thanthe maximum negative dispersion limit Neg_Disp_Limit of the network; (f)inserting the expanded entries into said data structure; and (g)repeating the steps (c) to (f) until the effective dispersions aresubstantially zero for the extracted entry in the step (c).
 2. A methodas described in claim 1, wherein the step of introducing andinitializing the data structure comprises introducing and initializingthe data structure which is a priority queue, including maintaining theentries in an ascending order according to the score; and the step (c)of extracting the entry with the lowest score comprises extracting thefirst entry from the priority queue.
 3. A method as described in claim2, further comprising the step of maintaining the priority queue in anascending order according to the score, comprising sorting the priorityqueue, being performed after the step (f).
 4. A method as described inclaim 2, wherein the step (f) comprises inserting the expanded entriesinto the priority queue so that the priority queue maintains theascending order according to the score.
 5. A method as described inclaim 2, wherein the step of determining the lightpath topologycomprises: identifying lightpaths in the network; assigning lightpathidentification numbers to the lightpaths; identifying fiber spans overwhich the lightpaths are laid.
 6. A method as described in claim 5,wherein the step of identifying lightpaths in the network comprisesidentifying all lightpaths in the network including protectionlightpaths and reconfigurable lightpaths.
 7. A method as described inclaim 2, wherein the step (e) of calculating the score comprisescalculating the score to be equal to the sum of: the total value of theDCMs stored in the entry; and the remaining effective dispersion in thenetwork divided by the number of lightpaths having remaining effectivedispersion.
 8. A method as described in claim 2, wherein the step (e) ofcalculating the score comprises calculating the score to be equal to thesum of: Σw_(i)·DCM_(min), wherein DCM_(min) is the smallest DCM value tobe used in the network, and w_(i) is the weight factor for thecorresponding DCM_(i) stored in each expanded entry; and${{g(x)} \cdot {\min \left( \frac{w_{i} \cdot {DCM}_{\min}}{{DCM}_{i}} \right)}},$

wherein g(x) is the remaining effective dispersion in the networkdivided by the number of lightpaths having remaining effectivedispersion, and$\min \left( \frac{w_{i} \cdot {DCM}_{\min}}{{DCM}_{i}} \right)$

is the smallest value of$\left( \frac{w_{i} \cdot {DCM}_{\min}}{{DCM}_{i}} \right).$


9. A method as described in claim 2, wherein the step (c) of determiningif the effective dispersions are substantially zero comprises measuringthe effective dispersions in units of distance.
 10. A method asdescribed in claim 2 further comprising the step of determiningalternative DCM locations such that the transfer of the DCM to thealternative location does not change the accumulated dispersion alongany lightpath in the network.
 11. A method as claimed in claim 1,wherein the step of introducing and initializing the data structurecomprises introducing and initializing the data structure, which is aseries of priority queues, the series being maintained in an ascendingorder according to the score of the first entry of the priority queues,and the step (c) of extracting the entry with the lowest score comprisesextracting the first entry from the first priority queue in the seriesof priority queues.
 12. A method as described in claim 1, the methodproviding the optimal location and value of one or more dispersioncompensating modules (DCMS) in an optical network, the optimal locationbeing defined as providing one or more of the following: optimal totaldispersion in the network; minimal number of DCMS in the network; andminimal cost of DCMs in the network.
 13. A method as described in claim1, wherein the step (d) of expanding comprises expanding the extractingentry into multiple entries by adding every available combination of DCMlocation and value to the extracted entry.